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Hi Steve
I too have experimented with home-made focal reducers. I currently have one fitted to my guiding camera to reduce the focal length of my guiding scope from 1000 to about 660mm (ie focal reduction of 0.66). I trust that you already know that:
Focal reduction ratio = 1 - distance of lens centre to the CCD focal plane divided by the focal length of the lens.
I had a dinky finderscope that came with a scope that I bought. However, it did have a nice glass objective lens. I found that the lens had a focal length of 85mm and buy popping out the lens in a cheap screwon Moon filter the little scope lens fitted exactly in the holder.
I then measured the length from the screwed on holder to the ccd as 29mm. Using the above equation 1-29/85 = focal reduction of 0.66.
I do not used this focal reducer for imaging but I do note that the stars showing in my guiding camera are perfectly round with no coma or "rooster
tails".
Cheers Peter
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